This invention relates to energy transmission means. More particularly, this invention relates to the detonation of explosives. In still another aspect, this invention relates to a novel, low energy transmission means for transferring an explosive signal from the blaster to the remote location of a receptor blasting cap, or to a signal time delay element, or to a signal relay element, or the like.
There are three major methods of igniting detonators which are used, for example, by the mining industry. They are: electric ignition, powder fuse ignition and ignition by means of a detonating cord.
In commercial mining, quarrying, tunneling and shaft work, the most popular and widely used method of blast initiation involves the use of electric blasting caps. Electric blast initiation is considered by most to be the safest method since it enables the blaster to electrically check all blasting caps before, as well as after they are loaded into the blasting site, such as a borehole. The whole, or any part of the electric blasting circuit can be checked with an approved blaster's galvamometer or an approved blaster's multimeter. The probability of encountering unexploded explosives in, for example, a muck pile, is greatly reduced. Risk of injury from accidentally digging into the explosives is also greatly reduced. In the electrical ignition method, each detonator is ignited by means of an electric current transmitted through insulated wires and generated by a current source placed at a safe distance from the explosive. The advantage of this method is that precise timing of detonation is possible facilitating the highly coordinated ignition of a series of charges. However, there are those who feel that the advantages of electric ignition are outweighed by the potential for inadvertent energizing of all or part of the electric blasting circuit by extraneous electricity.
The powder fuse ignition system ignites the detonator by combustion which is initiated at a safe distance and propagated along the train of powder to the detonator. Because of the relatively slow rate of combustion and variation in the rate caused by uneven distribution of the powder, powder fuse ignition systems do not provide an adequate means of ignition where short itervals between initiation and detonation are required.
The third method for igniting detonators is the detonating cord method which involves the propagation of the detonation energy along the cord to the detonating device. In order to insure propagation of the detonation energy to the detonating device or explosive, a conventional detonating cord usually contains between 4 and 400 grains of high explosive per linear foot. The explosive is typically PETN, RDX or TNT having a bulk density greater than 1.0 gram per cubic centimeter and a detonation velocity of about 20,000 feet per second. The high density and high detonation velocity of these materials produce a high brisance detonation which is capable of initating most cap-sensitive explosives. A major disadvantage of this conventional detonating cord is that the side blasting which necessarily results from its use may give rise to undesirable, or premature, detonation of explosives other than those intended to be detonated. For example, if a length of conventional detonating cord is placed in a borehole alongside an explosive charge, with the object of obtaining bottom hole initiation, it frequently occurs that the side blast of the detonating cord is sufficiently intense to initiate the main charge in the upper portion of the borehole resulting in poor rock breakage. If, in an effort to avoid this problem, a relatively insensitive blasting charge is used in place of a cap-sensitive charge, the explosive may not be initiated by the detonating cord but is frequently compressed to a state of insensitivity by the powerful blast of the detonating cord. Under these circumstances, the main charge may fail to detonate at all or may be partially detonated or detonation may occur at a reduced velocity.
When conventional type detonating cord is used above ground, its excessive power causes noise and air blasts which are unacceptable in populated areas and may cause injury because of flying debris.
A low energy detonation tube has been disclosed in U.S. Pat. No. 3,590,739 which attempts to solve the problem of excessive brisance by leaving the tube hollow and applying only a thin coating of explosive dust on its inner wall. Upon initiation, a detonating wave is generated which travels through the hollow tubing. One main disadvantage of this device is that a bend, kink, knot, crimp or cut in the tubing can sometimes stop propagation of the detonating wave. Also, unequal distribution of the explosive as a result of flaking may lead to dangerously high local concentrations of the explosive at some points in the tube.
Thus, a need has arisen for a detonating cord or energy transmission device which has a low brisance so as to prevent non-intended detonations and other accidents due to side blasts. At the same time, it is desirable that such device develop enough detonating force to pass through minor barriers or air gaps which might occur due to crimping, kinking or bending of the cord, and to eliminate the possibility of the explosive settling within the tube.